1. Field of the Invention
The present invention relates to a thermal flow sensor for measuring a flow rate by means of hot wires.
2. Description of Related Art
Some conventional thermal flow sensors for measuring a flow rate by using hot wires are provided with a measuring chip manufactured by a semiconductor micro-machining technology as a sensor part as shown in FIG. 18. In a thermal flow sensor 101 in FIG. 18, a gas to be measured is caused to flow in an inlet port 102, is made into a laminar flow by a laminar flow mechanism 103, and is caused to pass through a measuring passage 104 and flow out through an outlet port 105. For measurements of the flow rate of the gas, a measuring chip 111 connected to an electric circuit 106 is arranged in an exposed state in the measuring passage 104.
In this regard, the measuring chip 111 is constructed, as shown in FIG. 19, of a silicon chip 116 on which an upstream thermo-sensor 112, a heater 113, a downstream thermo-sensor 114, and an ambient temperature sensor 115 (these sensors 112 through 115 correspond to xe2x80x9chot wiresxe2x80x9d) and others are formed by a semiconductor micro-machining technology.
Accordingly, in the thermal flow sensor 101 in FIG. 18, while the gas does not flow through the measuring passage 104, the distribution of temperatures of the measuring chip 111 in FIG. 19 is symmetrical with respect to the heater 113. While the gas flows through the measuring passage 104, on the other hand, the temperature of the upstream thermo-sensor 112 is decreased and the temperature of the downstream thermo-sensor 114 is increased. Accordingly, the distribution of temperatures of the measuring chip 111 becomes unsymmetrical according to the flow rate of the gas to be measured. At this time, the degree of such unsymmetry is outputted as a difference in resistance values between the upstream thermo-sensor 112 and the downstream thermo-sensor 114. Thus, the flow rate of the gas can be measured by the electric circuit 106.
In the thermal flow sensor 101 in FIG. 18, however, six electrodes D1, D2, D3, D4, D5, and D6 are provided on the silicon chip 116 of the measuring chip 111 in FIG. 19 so that the upstream thermo-sensor 112, the heater 113, the downstream thermo-sensor 114, and the ambient temp. sensor 115 are individually connected to the electric circuit 106 by wire bonding using those six electrodes D1-D6.
In the thermal flow sensor 101 in FIG. 18 as above, the measuring chip 111 is exposed in the inside of the measuring passage 104 where a bonding wire W is provided. When a high flow rate of a gas to be measured is caused to flow in the measuring passage 104, accordingly, the bonding wire W can possibly be cut due to the pressure of the flowing gas. To prevent it, addition of a covering mechanism (for example, xe2x80x9ca support body 13axe2x80x9d in Japanese patent unexamined publication No. Hei 10-2773) or other measures must be taken.
The present invention has been made in view of the above circumstances and has an object to overcome the above problems and to provide a thermal flow sensor which uses a measuring chip as a sensor part, the measuring chip being provided with hot wires, and which can eliminate the use of wire bonding for connection between the hot wires of the measuring chip and an electric circuit.
The thermal flow sensor according to the present invention made to achieve the above purpose is characterized by including: a measuring chip provided with a hot wire and an electrode for the hot wire, the electrode being connected to the hot wire; a board having a front side on which an electrode for an electric circuit is provided, the electrode being connected to the electric circuit for performing a measuring principle using the hot wire; a body to which the measuring chip is attached in close contact relation to form a main passage; wherein at least one of the measuring chip and the board is formed with a groove, and a sensor passage in relation to the main passage is formed by the groove between the measuring chip and the board in a state where the measuring chip is mounted on the board with the hot wire electrode being joined to the electric circuit electrode, and the hot wire is laid across the sensor passage.
The thermal flow sensor according to another aspect of the invention is characterized a thermal flow sensor including: a measuring chip provided with a hot wire and an electrode for the hot wire, the electrode being connected to the hot wire; an electric circuit electrode pin connected to an electric circuit for performing a measuring principle using the hot wire; a board in which the electrode pin is inserted; an elastic body which provides a seal between the electric circuit electrode pin and the board; and a body in which the board is attached in close contact relation to form a main passage; wherein a sensor passage in relation to the main passage is formed in a long slot shape by a thickness of the elastic body between the measuring chip and the board in a state where the measuring chip is mounted on a front side of the board with the hot wire electrode being joined to a flat head of the electric circuit electrode pin, and the hot wire is laid across the sensor passage.